SQLite
Essence
SQLite is an embedded relational DBMS: no server, the whole database is a single file, and the engine links into the app (import SQLite3; it ships with iOS). It supports ACID transactions, indexes, triggers, FTS5 (full-text search) and JSON functions. It underlies Core Data and SwiftData; directly it's used through the SQLite3 C API or wrappers such as GRDB.
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